The invention concerns a thermoplastic monofilament for bristles which are exposed to extensive moisture during use.
Bristles of thermoplastic plastic materials are used for the most different types of brushes, paint brushes and the like. They are subjected to greatly differing physical and chemical loads due to their functional use for brushing, painting, polishing, application of media or the like, and owing to the conditions prevailing directly during use or at the location of use. These conditions include in particular the use of auxiliary agents such as detergents, cleaning agents, cosmetics, water or the like. Should the location of use have high ambient humidity, the drying of moist auxiliary agents is thereby precluded.
For this reason, bristles must have properties which meet these highly differing requirements. These different requirements are illustrated and exemplified by a toothbrush, wherein the bristles have an extremely small diameter of between approximately 0.05 and 0.4 mm and are disposed at a small separation from each other and also densely packed into bundles. Toothbrush bristles must have good bending properties to be able to bend within the purely elastic range and to return to the initial position without deformation (bend recovery). Due to the plurality of different movements and differing user-dependent individual application of pressure for brushing her/his teeth, the bristles must have a good fatigue strength under reversing bending stresses as well as long term stability for an appropriate period of use. The plastic material must be structured such that the bristle can be processed in mechanical or thermal production procedures, in particular for mounting the bristles to the bristle carrier. Finally, the bristles must be rounded at the useful end to produce protective and at the same time sufficient brushing action on the tooth surfaces and in the interdental spaces without damaging the enamel due to excessive hardness, edges or the like and without injuring the gums.
Due to the required dense bristle stock of a toothbrush, the auxiliary agents used, e.g. tooth paste, water etc., as well as the excessive moisture in the air at the location of use, e.g. bathroom, the bristles are constantly subjected to moisture. Drying proceeds slowly due to the dense bristle stock and the ambient conditions.
As has been previously noticed with earlier natural bristles, the excessive moisture weakens the bristles. Such failure was referred to as “wet breakdown” (U.S. Pat. No. 2,309,021). With natural bristles, this failure is mainly due to the loss of natural fat during washing and cleaning processes, the fat serving a hydrophobic role for the living animal. This process in a toothbrush is further enhanced by hydrolytic interaction between tooth cleaning agents and the bristle and by abrasive particles in the tooth cleaning agents. Attempts had been made to overcome this “wet breakdown” through surface chemical treatment of the natural bristle.
For the above reasons, the natural fibers of toothbrushes could only be replaced by synthetic plastic materials when high-quality thermoplastic materials became available. Today, bristles for toothbrushes exclusively use high quality polyamides, in particular PA 6.10 or 6.12. Polyamides of such quality are correspondingly expensive, in particular since these high-quality polyamides are too expensive for other applications.
Although high-quality polyamides have excellent strength properties, they absorb significant amounts of moisture thereby compromising their favorable strength characteristics. This reduction in strength leads to failure of the bristle, the earlier the smaller its diameter, since the moisture rapidly penetrates into the bristle core. Today, toothbrushes are often characterized on the basis of bristle hardness, e.g. soft, medium or hard, which is mainly determined by the bristle diameter. This characterization and grading looses its validity after a period of use due to the influence of moisture. The absorption and retention of moisture in the bristle gaps also leads to considerable hygienic problems since the moisture promotes bacterial growth which would require even longer drying times that cannot be achieved in practice (U.S. News & World Report 12.10.87, page 88).
Many other types of thermoplastic monofilaments have been repeatedly tried and tested for different kinds of bristles. In particular, bristles of thermoplastic polyesters have a comparable bending behavior and fatigue strength under reversing bending stresses but have not yet been successful for toothbrushes. This is attributable to their increased hardness and resulting danger of injury to gums and enamel. Moreover, they tend to split and form ridges during mechanical processing, in particular during rounding of the useful ends. In addition, they have an increased tendency of binding dirt, i.e. are difficult to clean by rinsing of the toothbrush.
Co-extruded filaments are known from textile technology (EP 0 763 611 A1) which consist of a polyester core and a polyamide jacket. The requirements on such applications are, however, different from those for bristles. Co-extruded monofilaments for toothbrushes are also known (WO97/14830) which consist i.a. of a polyamide jacket and a polyester core. Only high-quality polyamides are used for the jacket. In a further variant with a jacket of polyamides of lower quality, the core consists of a copolyether ester which is an elastomer and therefore does not meet the requirements for toothbrush bristles. The same is true for another known composite structure (GB 980 814), having a core and jacket made from differently treated polyvinyl chlorides.
It is the underlying purpose of the invention to propose a monofilament for bristles whose properties can be optimally adapted to the desired functional, technical requirements and ambient conditions during use and at reduced material cost.